A new approach for studying dietary obesity and its metabolic consequences is adopted in the proposed study. Rats will be fed, or overfed, intragastrically via chronically implanted cannulas. This procedure allows the rats free movements in the cages while being chronically fed. Liquid diets have been developed allowing flexibility in volumes and macronutrient compositions. Using this approach, behavioral and metabolic aspects related to the etiology and consequences of dietary obesity will be examined. The study will attempt to provide answers to the following questions: 1) Dose hyperphagia on a high fat diet result merely from the hedonic properties of the diet or from metabolic consequences of its consumption? 2) In which way does the level of surfeit feeding and diet composition affect the course of adipose tissue growth? 3) Is adipose tissue lipoprotein lipase likely to play a causative role in the development of obesity? 4) How precise is the control mechanism of food intake in the aftermath of an episode of obesity, and how are parameters of the adipose tissue (i.e., its mass, fat cell size and number) related to food intake behavior? 5) To what extent does a gain in body weight affect voluntary physical activity? 6) In there a dietary induced thermogenesis? What does it amount to? To what extent is it affected by diet composition? In the proposed study, Wistar Lewis rats, known to grow at a normal rate and to become obese on a high fat diet, will be compared to S5B/pL rats, which have a slow growth rate and show only limited responsiveness to a high fat diet. The comparisons will include aspects of the control mechanism of food intake, adipose tissue growth and energy expenditure, and may provide clues on a metabolic make-up which protects the S5B/pL rat from gaining weight and becoming obese.